apps/bttester: Fix BTP MTU size
apps/bttester: Fix BTP MTU size

GATT/CL/GAW/BI-09-C sometimes requires us to receive
>1024 bytes of data.

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble